Vacuum type electric circuit interrupter

ABSTRACT

A vacuum-type circuit interrupter comprises separable contacts, each comprising a body of iron having one or more recesses therein and a material within the recesses consisting essentially of copper and bismuth, the bismuth being present in an amount of a few percent or less by weight of the material. When the interrupter is first operated, the arcing surface of each contact is constituted by portions of copper-bismuth and by portions of iron.

United States Patent Amoth et al. Jan. 1, 1974 [54] VACUUM TYPE ELECTRIC CIRCUIT 3,246,979 4/1966 Lafferty et al. 200/144 B INTERRUPTER 3,327,081 6/1967 Pflanz 200 144 B [75] Inventors: Victor W. Amoth, Media, Pa.;

Graham R. Mitchell, Willingboro, Primary Examiner-Robert S. Macon N.J.; Carl C. Popadick, West Attorney-J. Wesley Haubner et al. Chester, Pa.

[73] Assignee: General Electric Company,

Philadelphia, Pa. [5 ABSTRACT [22] Filed: 1972 A vacuum-type circuit interrupter comprises separable [2H AppL NOJ 2 5 contacts, each comprising a body of iron having one or more recesses therein and a material within the recesses consisting essentially of copper and bismuth, [52] US. Cl 200/144 B, 200/166 C the bismuth being present in an amount of a few [51] hit. Cl. HOlh 33/66 cent or less by weight of h material. w the inter [58] Field of Search 200/144 B, 166 C rupter is first Operated the arcing Surface of each contact is constituted by portions of copper-bismuth [56] References C'ted and by portions of iron.

UNITED STATES PATENTS 2,900,476 8/1959 Reece 200/144 B 6 Claims, 3 Drawing Figures PATENTEDJAH 1 I974 VACUUM TYPE ELECTRIC CIRCUIT INTERRUPTER BACKGROUND This invention relates to a vacuum type electric circuit interrupter and, more particularly, to contacts for such an interrupter that have an exceptionally high resistancc to arc-erosion and to contact-welding.

In the usual vacuum type circuit interrupter, there are a pair of contacts that are movable into and out of engagement. When the contacts are moved out of engagement, a gap is established therebetween across which an arc is developed. The surfaces of the two contacts that border this gap are referred to hereinafter as the arcing surfaces of the contacts.

Each of the contacts of our interrupter comprises a body principally of iron having one or more recesses therein that are filled with a principally copper material. Before any arcing has occurred between the contacts, the copper material in the recesses and the iron material of the body have exposed surfaces which constitute the arcing surfaces of the two contacts.

Both copper and iron have a strong tendency to weld together when used for the contacts of a vacuum interrupter. In US. Pat. No. 3,246,979-Lafferty et al., it is pointed out that contact-welding can be inhibited in a given base metal by adding to it in the liquid state a small percentage of a metal which is soluble in the base metal in the liquid state, but insoluble in the solid state. The additive, bismuth, for example, satisfies these requirements with respect to copper base metal. Bismuth does not, however, satisfy the solubility requirements with respect to iron as a base metal because bismuth is essentially insoluble in iron in the liquid state. This insolubility in the liquid state makes it very difficult to get a good dispersion of the bismuth in the liquid iron. If the bismuth is simplyadded to the iron when the iron is in its liquid state, then upon solidification, the two metals will solidify in two separate layers, producing an essentially unusable contact material. To obtain a satisfactory dispersion, complicated and expensive mixing techniques are needed.

SUMMARY An object of our invention is to use iron as one of the contact materials on which contact-engagement occurs and to inhibit welding between the iron portions of the contacts with a bismuth additive without the necessity for making an iron-bismuth alloy'by means of complicated mixing techniques.

In carrying out our invention in one form, we provide, in a vacuum type circuit interrupter, separable contacts each comprising a body of iron having one or more recesses therein containing a material consisting essentially of copper and bismuth, the bismuth being present in an amount of a few per cent or less by weight' of the material. The bismuth is incorporated solely in the copper, and no effort is made to incorporate it in the iron. When arcing occurs between the contacts during an interrupting operation, the are or arcs vaporize both the copper-bismuth and the iron. The vapors rapidly mix, and soon afterwards the vapor mixture condenses out on the contacts as a copper-iron-bismuth alloy in a film covering substantially the entire arcing surface. The copper component of this alloy serves to disperse throughout the alloy the small amount of bismuth present, thus making the bismuth available to act as a weld inhibitor wherever a weld may occur on the arcing surface. The copper-iron-bismuth alloy which covers the arcing surface is a hard material which has an exceptionally high resistance to arc-erosion and contact-wear.

BRIEF DESCRIPTION OF DRAWINGS For a better understanding of the invention, referonce may he had to the following description taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a side elevational view partly in section showing a vacuum interrupter embodying one fonn of the invention.

FIG. 2 is a plan view of one of the contacts present in the interrupter of FIG. 1.

FIG. 3 is an enlarged sectional view along the line 3-3 of FIG. 2.

SPECIFIC DESCRIPTION OF PREFERRED EMBODIMENT Referring now to FIG. 1, there is shown a vacuumtype circuit interrupter comprising a sealed envelope 11 evacuated to a pressure of 10 torr or lower. The envelope 11 comprises a tubular casing 12 of insulating material and a pair of metal end caps 13 and 14 suitably sealed to the opposite ends of casing 12.

Within the evacuated envelope 11 there are a pair of separable disc-shaped contacts 17 and 18. Contact 17 is a stationary contact brazed to the lower end of a stationary conductive contact rod 17a; and contact 18 is a movable contact brazed to the upper end of vertically movable conductive rod 180. The movable contact rod 18a projects freely through an opening in the lower end cap 14, and a flexible metal bellows 20 provides a suitable seal thereabout that allows vertical movement of the contact rod 18a without impairing the vacuum inside envelope 11.

All of the internal parts of the interrupter are substantially free of surface contaminants. These clean surfaces are obtained by suitable conventional vacuum processing, which involves baking out the interrupter during its evacuation.

Closing of the interrupter is effected by driving the contact rod 18a in an upward direction to drive movable contact 18 into engagement with stationary contact 17. Opening is effected by returning the movable contact from its engaged position downwardly to its solid line position shown. When the contacts are separated during an opening operation, an arc is drawn across the gap 19 therebetween. Assuming an altemating current, this arc persists until a natural current zero, at which time it vanishes and is prevented from reigniting by the high dielectric strength of the vacuum in the envelope 12. A suitable metal shield 21 of tubular form surrounds the contact to condense the metal vapors generated by the arc, thus assisting in the interrupting process.

The gap 19 that is established between the contacts when they are separated is referred to hereinafter as the arcing gap, and the surfaces of the two contacts bordering gap 19 are referred to as the arcing surfaces of the contacts.

The illustrated interrupter is intended for use in applications where it will be opened and closed frequently, and it is therefore important that its contacts have a high resistance to arc-erosion and contact-wear. For attaining this high resistance, we form each contact of a body 25 (FIGS. 2 and 3) of iron or steel. In a specific embodiment of the invention, the material of body 25 is vacuum-melted low-carbon iron, such as Carpenter Technology Co.s Consumet core iron, which contains a maximum of about one percent by weight of impurities, including a maximum of 0.06 percent carbon. This body 25 contains a plurality of recesses 26 and 28 filled with another material 30 principally of copper, which will soon be described in more detail. In a preferred form of the invention, recess 28 is a centrally disposed circular recess and recess 26 is an annular recess surrounding and spaced from central recess 28.

Assuming an interrupter that is new and whose contacts have not yet been exposed to arcing, the metal 30 in each of the recesses 26 and 28 terminates in a substantially planar exposed surface 32, and the body portion 25 terminates in a substantially planar exposed surface 34, all of which planar surfaces are substantially flush with each other.

When the contacts are driven into engagement during a closing operation, the iron body portions 25 of the two contacts will engage each other, and the copper base materials 30 in the recesses will also engage each other. To inhibit contact-welding between the copper portions, we form these portions of a material consisting essentially of copper and bismuth, where the bismuth is present in a few percent or less by weight, e.g., about 0.2 to three percent. The bismuth, being insoluble in the copper in the solid state, acts as a good weldinhibitor, as explained in the aforesaid Lafferty et al patent. The copper-bismuth alloy is easily formed by mixing the desired amount of degassed bismuth with degassed liquid copper. The liquid-state solubility of bismuth and copper enables these materials to readily mix, thereby assuring a good dispersion of the bismuth throughout the copper.

Bismuth, being insoluble in iron in the solid state, could also be a good weld-inhibitor for the iron if it could be thoroughly dispersed in the iron. But such dispersion is difficult to obtain because bismuth and iron are insoluble in the liquid state. As pointed out earlier, one of the objects of our invention is to inhibit welding between the iron portions of the contacts with a bismuth additive without the necessity for making an ironbismuth alloy by complicated mixing techniques. This we accomplish by utilizing the arc to vaporize both the copper-bismuth and the iron of the contacts. Because copper and iron have arc voltages that are very nearly equal, the are or arcs formed during interruption vaporize both the iron and the copper base material 30. The vapors rapidly mix and soon afterwards condense out on the arcing surfaces in the form of an adherent film of copper-iron-bismuth. The copper component of this alloy serves to disperse throughout the alloy the small amount of bismuth present, thus making the bismuth available to act as a weld-inhibitor wherever a weld may occur on the arcing surface.

In order to make available for mixing vapors of both the copper and iron materials, it is important to avoid the preferred arcing referred to by Reece in his U.S. Pat. No. 2,900,476. According to Reece, where the two materials exposed to a vacuum are have widely different arc voltages, the vacuum arc will seek out and vaporize the material with the lower arc voltage, a phenomena that we refer to as .preferred arcing. According to MD. Paul in his note entitled Vacuum Arc Voltage appearing in the journal Nature, Volume 215,

pages l,474l,475, Sept. 30, l967, published in London, England, iron and copper each have an arc voltage of about 15.5 volts. With such a combination, no preferred arcing is to be expected, and our tests do, in fact, appear to confirm this expectation. The are tends to evenly erode the copper material and the iron, and no evidence of preferred arcing on either material is apparent. (It is to be understood in the above discussion that the amount of bismuth present in the copper is so small that it has substantially no effect on the arc voltage of the copper and does not alter the approximately equal relationship of the copper and iron arc voltages.)

The mixing of the copper and iron vapors that results from the above described non-preferred arcing plays an important role in assuring that substantially the entire film that covers each of the arcing surfaces will be an alloy primarily of copper and iron. Such an alloy is a hard material (much harder than the original copper or iron materials) which has an exceptionally high resistance to arc-erosion and contact-wear. The film thickness builds up as the interrupter is operated and appears to reach a stable value of about 5 to 15 mils after around 100,000 or so interrupting operations, depending upon the current level, which in our tests was 700 amperes, r.m.s. Typically, many times this number of such operations can be sustained with contacts only 1 inch in diameter.

Contacts made as above described (where material 30 was copper-bismuth containing 0.2 percent bismuth by weight) have been shown in test to have an arcerosion rate less than about one-fifth of that of plain copper-bismuth contacts of about the same composition. The contacts remained exceptionally smooth after repeated arcing operations and were free of the sharp projecting whiskers which often appear on vacuum interrupter contacts under such conditions. These tests were conducted at 325 volts and 700 amperes, r.m.s.

Although the form of our invention particularly described hereinabove uses bismuth as the anti-weld additive, it is to be understood that other suitable additives, such as lead, which have the same solubility properties as bismuth with respect to copper and iron could be used. In this regard, lead, like bismuth, is substantially insoluble in iron in both the solid and liquid states, is insoluble in copper in the solid state, but is soluble in copper in the liquid state.

in referring to additives that are substantially insoluble in copper and iron in the solid state, we are referring to additives that have a solid-state solubility in copper and iron of less than about two percent by weight of the alloy considered at the freezing temperature of said additive. The term a few percent, as used herein, is intended to denote a percent less than five.

While we have shown and described a particular embodiment of our invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the invention in its broader aspects; and we, therefore, intend herein to cover all such changes and modifications as fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In a vacuum type electric circuit interrupter:

a. a sealed envelope evacuated to a pressure of 10" torr or less,

b. a pair of separable contacts disposed within said envelope and mounted for relative movement into and out of engagement with each other,

c. said contacts having arcing surfaces facing each other on opposite sides of a gap that is present between the contacts when disengaged,

d. each of said contacts comprising a body having at least one recess therein with portions of said body surrounding said recess,

e. said recess containing a material consisting essentially of copper and a second metal and having a surface constituting a portion of said arcing surface,

f. said body having a surface principally of iron and constituting another portion of said arcing surface,

g. said second metal being present in said recess material in an amount of a few percent or less by weight of said material, said second metal being substantially insoluble in copper and in iron in the solid state, being substantially insoluble in iron in the liquid state, but having substantial solubility in copper in the liquid state,

h. said contacts engaging each other on said body surfaces principally of iron.

2. The vacuum interrupter of claim 1 wherein said second metal is lead. 

2. The vacuum interrupter of claim 1 wherein said second metal is bismuth.
 3. The vacuum interrupter of claim 1 wherein said second metal is lead.
 4. The vacuum interrupter of claim 1 in which operation of said interrupter produceS on each contact a film of an alloy of copper, iron, and said second metal covering said surfaces of said recess material and said body bordering said gap and thereafter serving as the arcing and contact-making surface of said contact.
 5. The vacuum interrupter of claim 4 in which said second metal is bismuth.
 6. The vacuum interrupter of claim 4 in which said second metal is lead. 